Rashi on Deuteronomy 25:2/רש"י על דברים כ״ה:ב׳

1והפילו השפט AND THE JUDGE SHALL CAUSE HIM TO FALL DOWN — This teaches that one is not lashed whilst standing or sitting but only when bending (Makkot 22b).

2לפניו כדי רשעתו [AND THE JUDGE SHALL … SMITE HIM] IN FRONT ACCORDING TO HIS WICKEDNESS (כדי רשעתו may mean “as much as is sufficient for his wickedness”, i.e., for one part of his wickedness, רשעתו being singular) — in front (on his chest) corresponding to one part of his wickedness, and on his back corresponding to two parts: hence they (the Rabbis) derived that one must give him (anyone sentenced to lashes) two-thirds on his back and one third on his chest (Sifrei Devarim 286:6; Makkot 22b, 23a).

3בְּמספר BY A NUMBER — but it (the word) is not punctuated בַּמספר by the number; this shows that it is connected with the following word, to read בְּמִסְפַּר ארבעים [“he shall smite him”] … by the number of forty, (i.e. by the number leading to forty), not, however, a full forty, but a number that leads up to the full total of forty, i.e., forty less one (Makkot 22a).